The present invention relates to apparatus and methods for inspecting the surface of a substrate such as reticles, photomasks, wafers and the like (hereafter referred to generally as photomasks). More particularly, the present invention relates to an optical inspection system that can scan such a substrate at a high speed and with a high degree of sensitivity.
Integrated circuits are made by photolithographic processes, which use photomasks or reticles and an associated light source to project a circuit image onto a silicon wafer. The presence of defects on the surfaces of the photomasks is highly undesirable and adversely affects the resulting circuits. The defects can be due to, but not limited to, a portion of the pattern being absent from an area where it is intended to be present, a portion of the pattern being present in an area where it is not intended to be, chemical stains or residues from the photomask manufacturing processes which cause an unintended localized modification of the light transmission property of the photomask, particulate contaminates such as dust, resist flakes, skin flakes, erosion of the photolithographic pattern due to electrostatic discharge, artifacts in the photomask substrate such as pits, scratches, and striations, and localized light transmission errors in the substrate or pattern layer. Since it is inevitable that defects will occur, these defects have to be found and repaired prior to use. Blank substrates can also be inspected for defects prior to patterning.
Methods and apparatus for detecting defects have been around for some time. For example, inspection systems and methods utilizing laser light have been introduced and employed to various degrees to scan the surface of substrates such as photomasks, reticles and wafers. These laser inspection systems and methods generally include a laser source for emitting a laser beam, optics for focussing the laser beam to a scanning spot on the surface of the substrate, a stage for providing translational travel, collection optics for collecting either transmitted and/or reflected light, detectors for detecting either the transmitted and/or reflected light, sampling the signals at precise intervals and using this information to construct a virtual image of the substrate being inspected. By way of example, representative laser inspection systems are described in U.S. Pat. No. 5,563,702 to Emery et al., U.S. Pat. No. 5,737,072 to Emery et al., U.S. Pat. No. 5,572,598 to Wihl et al., and U.S. Pat. No. 6,052,478 to Wihl et al., each of which are incorporated herein by reference.
Although such systems work well, there are continuing efforts to improve their design to provide greater sensitivity and faster scanning speeds. That is, as the complexity of integrated circuits has increased, so has the demand on the inspection process. Both the need for resolving smaller defects and for inspecting larger areas have resulted in much greater magnification requirements and in much greater speed requirements, for example, in terms of number of pixels (picture elements) per second processed.
In view of the foregoing, there is a need for improved inspection techniques that provide increased scanning speeds.